1. Field of Invention
The present invention relates to mixer for use in electrical systems and more particularly to a mixer that has a relatively high linearity.
2. Description of Related Art
In the development of mixers for use in electrical systems, two types of loads have commonly been used for mixers based on Gilbert cells: resistive loads and MOSFET loads, where typically these loads can be included in an integrated circuit (IC). (Thomas E. Lee, "The Design of CMOS Radio Frequency Integrated Circuits", Cambridge, 1998. Behzad Razavi, "RF Microelectronics", Prentice Hall, 1998.)
In systems based on resistive loads, generally the gain of the mixer can only be increased by increasing the value of the resistors. Resistors with large values have higher parasitic capacitance associated with them and therefore can dramatically reduce the mixer speed and bandwidith. At the same time, large resistive loads combined with the relatively high bias currents required for high-speed operation can cause problems in the biasing of the Gilbert cell transistors and thereby effectively impose practical limits on the signal swing of the mixer output. Furthermore, the size of the resistive load may vary due to process variations by 30-40%, a variation that can change the gain substantially.
In systems based on MOSFET loads, many of the difficulties associated with resistive loads no longer constrain the design. However, the nonlinear voltage-current characteristics of MOSFETs can create signal harmonics at the output nodes so that the output of the mixer is degraded. For example, if two inputs of the mixer consist of the following ideal sinusoids for the radio frequency (RF) and local oscillator (LO),
RF: sin(.omega..sub.rf *t) PA1 LO: sin(.omega..sub.lo *t), PA1 IF: sin(.omega..sub.if *t) PA1 where .omega..sub.if =.omega..sub.rf -.omega..sub.lo.
then the resulting output includes the intermediate frequency (IF)
However, due to the nonlinear characteristics of MOSFET loads, the output will also contain components at odd multiples of the IF (i.e., sin(3*.omega..sub.if *t), sin(5*.omega..sub.if *t), . . . ). In general, for differential mixers the third harmonic will be much bigger in amplitude than other components. For a fixed input RF level, the ratio of the amplitude of the third harmonic to the amplitude of the desired IF signal (i.e., sin(.omega..sub.if *t)) is then a measure of the mixer's overall linearity. Making this ratio sufficiently small may pose substantial operational requirements on the mixer. Under nominal operating conditions, it is desirable to have the third harmonic be at least 35-40 dB below the desired IF signal.
Thus, the use of MOSFET loads in mixers based on Gilbert cells has many design advantages. However, the inherently nonlinear voltage-current characteristics of MOSFETs can lead to substantial performance limitations due to the creation of higher-order harmonics in the output.
Attempts to treat the nonlinear effects of MOSFETs have been developed in other contexts. For example, U.S. Pat. No. 5,717,362 (Patent '362) discloses an array oscillator circuit that includes a plurality of ring oscillators having a plurality of buffer stages for generating output signals. FIG. 16 of Patent '362 shows symmetric load elements 316 and 318, where symmetric load element 316 includes PMOS transistor 310 and diode-connected PMOS device 324 and symmetric load element 318 includes PMOS transistor 312 and diode-connected PMOS device 326. The biased PMOS device and the diode-connected PMOS device in each symmetric load element are sized to yield current-voltage characteristics that enable the buffer stage to achieve high dynamic supply noise rejection through a first-order cancellation of supply noise coupling. (Patent '362, column 12, line 64 to column 13, line 13.) This approach, while relevant to the technology of ring oscillators, is not directly applicable to mixers.